Foamed Roof Deck

I saw a house yesterday with a foamed roof deck. While talking to the HO and his wife, she got the invoice and the recommendation out. It recommended taking the old insulation out and the spraying 4 inches of foam.

If you take the old out, why foam the roof deck? Why not foam the attic side of the ceiling?

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I would like to point out one problem that I have seen with cc foam under the roof deck. Most roofs that I see are leaking to some extent, usually due to poor wall and chimney flashings. A vapor barrier on both sides of the sheathing is a bad idea, especially when you have fiberglass composite or cedar shingles since they are not waterproof. The underlayment is typically 15# felt, which is also not water proof. The only way to ensure that moisture is not driven into the sheathing, for these roofs, is to cover the entire roof deck with ice and water shield.

If you are going to do a hot roof retrofit, ensure the roof is perfect, then use a more vapor open material like cellulose or oc foam. The underlayment is the vapor barrier, so there is no need to have another on the inside.

Bill - amen on those recommending upgrades, but the issues you discuss with the roofs in your area (and it seems like quite a few others areas) are a poor excuse to shortchange codes, energy efficiency, & building science. He11 if they followed the codes which says to follow the bloody manufacturer's directions, you shouldn't be having those issues. Now if you're in 2 or 3 dry CZ you can get away with OC (as I recall parts of Utah are, so you might be safe saying that) - everywhere else it requires a vapor barrier sprayed on it, or you use CC foam.

Personally felt paper needs to come off the roofs, and be replaced with either I&W or a product like Titanium UDL. No matter what is used, make sure they do it properly (i.e. no staples, lapped & flashed properly, etc...) If you are going to do a hot roof, the roof needs to be done properly, the sheathing & structure needs to have already been dried out & the proper product installed - with this done, it doesn't matter if you have 2 vapor barriers (in case anything does fail though - I don't want a spounge up there)

I liked a lot about that article. Random, thoughtless improvements cause more negative unanticipated consequences that a more thoughtful and comprehensive approach.

Regarding painting foam with a vapor barrier. Everything I've read, and my intuition, tells me vapor barriers are a thing to avoid (wish I understood perm ratings better - I think there is probably a permeability sweet spot). I have a feeling impermeable moisture barriers are synonymous with unintended/unanticipated consequences.

Maybe the key is to have all materials have a perm within a certain range of each other so moisture, if present, moves at a consistent rate through components rather than backing up at one location.

If moisture drive or movement of any type makes it's way into a building component, I think you want it to be able to move through and out rather than being dramatically slowed/stopped at the boundary of 2 materials.

So you don't confuse the building by having 2 thermal barriers on the upper side of your six sided box. Like insulating the attic floor of an 10 foot ceiling home, installing a dropped ceiling and then putting insulation on the dropped ceiling.

There are two general types of foamed roof - hot or vented, but the end result is the attic is brought into the conditioned space. David Meiland likes to say semi-conditioned space which is a better description as some inspectors, HVAC guys, etc... think that you need to add ducting to it. While that is a side point, it is relevant to your question

While insulation is good and generally more is better, placing it in that location doesn't do you any good. The thermal boundary is now the roof, not the ceiling anymore. But won't the attic heat up still - no not really, if done right you are looking at maybe 3 - 5 degrees difference which is about the same as a 2 story house with high ceilings.

I'm going to differ with those who want just one specific insulation layer. I think leaving insulation in place on the flat after redefining the primary boundary to the roof deck makes this large "unintentionally conditioned" space into a huge additional insulation or temperature loss buffer.

After all, air is the insulating factor, not the material.

Heat goes to cold, right? Bigger the delta, faster it moves, right? So wouldn't heat move from 70 to 50, then 50 to 20 a LOT slower than going straight from 70 to 20?

Of course you need to be cognizant of issues with dew point. And some may argue removing fiberglass removes a health hazard. But for purposes of energy loss, I think the more insulation between intentionally conditioned and outdoors the better.